The subject matter of the present invention is a method of preparing o-, m- and p-bis-(chloromethyl)-tetrachlorobenzenes of the general formula ##STR2## characterized in that o-, m- or p-xylene is chlorinated radically on the side-chains in the presence of hexamethylenetetramine (HMTA) in a one-pot process, after completion of an iron chloride-catalyzed nuclear chlorination.
In Houben-Weyl 5/3, p. 740, there is described a method of preparing 1,4-bis-(chloromethyl)-2,3,5,6-tetrachlorobenzene from p-xylene; here the method is practiced in nickel autoclaves using liquid chlorine at 12 to 15 atmospheres gauge pressure. For the technical preparation of this compound, nickel autoclaves are quite expensive, and chlorine under this pressure always constitutes a source of danger.
The chloromethylation of tetrachlorobenzene with bis-(chloromethyl) ether in the presence of fuming sulfuric acid is suitable only on a laboratory scale (Z. L. Vejdelek and M. Protiva, Collection Czechoslov. Chem. Commun. 39 (1974) 6, 1596). Also poorly suitable for industrial purposes is the preparation of the three isomeric bis-(chloromethyl)-tetrachlorobenzenes from the dioxonium compounds of tetra-(nucleus)-chloroxylenes with carbon tetrachloride and water (German "Offenlegungsschrift" No. 2,211,735).
The chlorination of xylene with chlorine gas in a one-pot process, i.e., the chlorination of the nucleus as well as the side chains in the same reaction vessel, is particularly suitable for the economical, technical preparation of bis-(chloromethyl)-tetrachlorobenzenes as in German Pats. 1,568,607 and 2,358,949. In the process of the first of these two patents, the nuclear chlorination is performed in a first step with a small amount of ferric chloride catalyst (0.05 to 1.0 wt.-% with respect to xylene), so as not to excessively impair the side-chain chlorination in the second step, the overall result being poor utilization of the chlorine, with poor yields and low purity.
In a criticl departure from this method, German Pat. No. 2,358,949 proposes the use of ultraviolet light to activate chlorine gas in the gas phase over the reaction solution, and then to agitate the surface of the reaction solution intensely to enable it to absorb the activated chlorine gas.
However, this method also has its disadvantages: A large surface is achieved only by intense agitation and a relatively wide reaction vessel; high-powered stirrers consume energy, and nowadays space-saving, slender reaction vessels of great useful capacity are preferred in the art. Furthermore, the emerging hydrogen chloride gas always carries with it a more or less great amount of chlorine gas, and it is desirable that the chlorination take not too long a time. In this process, however, the excitation of the chlorine gas in the second step by means of radical initiators is difficult or impossible. It has been found that the presence of iron chloride catalysts greatly impairs or destroys the effectiveness of the radical catalysts added in the second step.